Apneic episodes seem to be associated with increased sympathetic nervous system activity. (J. Hedner et al. Is high and fluctuating sympathetic nerve activity in the apnea syndrome of pathogenicimportance for the developement of hypertension? J Hypertens 1988). Epinephrine sensitizes human platelets in vivo and in vitro as studied by fibrinogen binding and P-selecting expression (Paul Hjemdahl, Nicholas A. F. Chronos, Darren J. Wilson, et al Arteriosclerosis and Thrombosis, Vol 14, No 1 January 1994). The aim of this study is to investigate if patients with sleep apnea have a higher morning increase in platelet activation and circulating catecholamines than patients without sleep apnea. Blood samples are drawn twice by venipuncture from every patient who is participating in the study. The first blood sample is taken at 9 pm; the second, at 6 : 30 pm AM the following morning. Irculating plasma- catecholamine, serum-cortisol, and P-selectin expression are analyzed on every blood sample. Serum-cholesterol level is analyzed on the morning sample. All patients with variable primary diseases routinely referred to the New England Medical Center's Sleep Lab by their physicians to have an all night sleep study are enrolled in the study if: 1) they are at least 18 years old and 2) give written informed consent. About 80 subjects (men and women) are expected to participate in the study over a period of about 40 months. The analysis will consist of a series of descriptive statistics on sociodemographis and clinical variables and two-group comparisons. We will compare the data for patients with and without sleep apnea dusing Chi- square or two-tailed Ficher's exact test for categorical variables and student's t-test for continuous variables.